Research will be undertaken in the following three areas related to the quantitative characterization of the electrical sources generated by the heart. (1) Cardiac tissue is described by uniform three dimensional intracellular space coupled everywhere (through the limiting membrane) to a uniform three dimensional interstitial space. Each space is anisotropic with a different conductivity tensor, in general. Based on both potential theory and Hodgkin-Huxley membranes a continuation of the study of activiation and the concomitant cardia sources is planned. This "bi-domain" model will also be used to evaluate measurements made to establish the passive, macroscopic, anisotropic conductivity parameters. (2) The effect of the discrete nature of cardiac tissue is to be examined particularly the electrophysiological role of junctions. This study is a continuation of one now being completed for one dimension, with extensions to two dimensions, and possibly three dimensions. The model structure is based on histological evidence. (3) A macroscopic cardiac source model is being developed that will characterize the space-time source of elemental tissue (each element contains a moderate number of cells). The model will contain conduction tissue and will generate isochrones and body surface potential maps.